1. Field of the Invention
The present invention relates to a comparator circuit to compare two input data and particularly relates to a comparator circuit embodied with a small number of elements.
2. Description of the Prior Art
Comparator circuits are commonly used in electronic circuits including those for A/D converters. With recent progress of high definition TVs and other sophisticated devices, higher precision A/D converters are required, and comparator circuits are also desired to have higher precision.
FIG. 4 shows an example of a conventional comparator circuit. This circuit was given in IEEE Journal of Solid-state Circuits (Vol. 3, No. 6, December 1988, Page 1346).
Referring to FIG. 4, this comparator circuit has a two-stage configuration with a differential pre-amplifier on the front stage and a comparison/latch circuit on the rear stage. At the differential pre-amplifier on the front stage, input voltage V.sub.IN and reference voltage V.sub.ref are, via an input buffer, input to the bases of the first and second NPN transistors Q1 and Q2, which constitute a differential pair so that the potential difference (V.sub.IN -V.sub.ref ) is amplified.
The amplified signal is input to the comparison/latch circuit on the rear stage. The input signal is, via an input buffer, input to the bases of the third and fourth NPN transistors Q3 and Q4, which constitute another differential pair. When the comparator driving signal .phi. is at the logical level "H", the input and the reference voltages are compared. If V.sub.IN is larger than V.sub.ref, the output Q.sub.N becomes "H" and the output Q.sub.R becomes "L". Then, when the comparator driving signal .phi. becomes "L", the latch signal (inverse of the signal .phi.) changes from "L" to "H" and keeps the same output.
The comparator circuit of FIG. 4 comprises 22 NPN transistors and 12 resistors. The number of comparator circuits needed for a parallel type A/D converter with N bits is 2.sup.N -1. If N=10, 1023 comparator circuits are needed. In other words, the higher the resolution improves, the larger the required number of comparator circuits becomes.
When a large number of comparators are needed for a higher resolution, conventional comparator circuits with many elements as described above involve drawbacks of large chip size and increased current consumption.